1. Field of the Invention
The invention relates to a method of preparing filled, modified and fiber reinforced thermoplastics by mixing polymers, blend polymers, possibly additives, fillers and fibers, and to a twin screw extruder for putting the method into practice.
2. Background Art
A method of the generic type is known, in which cut glass fibers of an average length of 3 to 6 mm are fed to an extruder by means of so-called lateral feed. These fibers are cut to an average length of 0.3 to 0.6 mm when mixed in. Fundamentally, the advantage of the mixed in fillers resides in that the finished product will be less expensive; as a rule, fillers also result in an increase of the elastic modulus of the thermoplastic and in the improvement of the dimensioned accuracy of components made from the thermoplastic by injection molding. As a rule, blending is employed to improve the free flowing properties of the thermoplastics and to make them more resistant to temperature and chemicals as well as to improve the mechanical properties such as impact strength.
It is further known to add fibers in the form of fiber sections of an average fiber length of more than 1.5 mm to thermoplastics. However, thermoplastics of this type are not sufficiently true to size for some fields of application, moreover, the impact strength leaves much to be desired. Additionally, temperature- and shear-sensitive polymers decompose during repeated melting and working processes, for instance during the subsequent incorporation of fillers.
U.S. Pat. No. 5,110,275 teaches an extruder to which glass fibers are added in the form of a fiber strand, a so-called fiber roving, in such a way that sufficient wetting with molten plastics takes place in the extruder.
U.S. Pat. No. 5,879,602 teaches an impregnating device in which the fiber rovings to be added to the extruder are impregnated with plastics.
The known extruders have in common that the fibers are broken into so-called short fibers, i.e. to very short pieces of 0.2 to 0.7 mm of length.
A so-called pulltrusion method is known from EP 0 056 703 B2, in which continuous fiber strands are pulled through a melt of plastics for a composite material to be formed. In this case, the fibers are maintained over their full length; but this method can only be used in the case of very low-viscous plastic melts and can therefore be applied only to a limited extent.
It is an object of the invention to embody a method of the generic type such that the thermoplastics produced possess high mechanical strength and high impact strength as well as splinterproofness upon rupture and high dimensioned accuracy during injection molding and to provide a twin screw extruder for putting the method into practice.
According to the invention, this object is attained by the following steps: a twin screw extruder is provided, having shafts that are drivable to rotate in the same direction, a conveying direction, a feed zone, a melting zone, a fiber feed zone, a fiber incorporation zone, a vent zone and a discharge zone; polymer and blend polymer are metered and fed to the feed zone; polymer and blend polymer are melted and mixed in the melting zone; filler is fed into the filler feed zone; polymer, blend polymer and filler are mixed and kneaded in the filler incorporation zone; fiber rovings are fed into the fiber feed zone; the fibers are cut to long fiber sections of at least 1.0 mm of average length and incorporated in the fiber incorporation zone; degassing by vacuum takes place in the vent zone; and the filled, modified and long-fiber reinforced thermoplastic is discharged in the discharge zone. The combination of the phases of the method specified therein helps produce thermoplastics which, upon injection molding, will give a product of special dimensioned accuracy, high mechanical strength and high impact strength as well as splinterproofness upon rupture. This is effected in particular by the long fiber sections. So far, it has not been possible to produce this thermoplastic in a single-step process. The single-step process according to the invention also ensures the use of polymers of special temperature and shear sensitivity such as polyester. Bumpers for vehicles can for example be made from thermoplastics of this type.
The extruder employed for putting the method into practice is distinguished in particular by a casing; shafts which are drivable to rotate in the same direction and which are parallel to each other in the casing and which define a conveying direction; screw elements and kneading disks which are mounted on the shafts; a feeder port for polymer and blend polymer; a melting zone provided downstream of the feeder port seen in the conveying direction; a filler incorporation zone provided downstream of the filler feed device seen in the conveying direction; at least one inlet port for fiber rovings which is provided downstream of the filler incorporation zone and opens into a fiber feed zone; a fiber incorporation zone which is formed downstream of the fiber feed zone and in which a pair of mixing elements and conveying screw elements are mounted alternately on the shafts; a vacuum vent zone which is provided downstream of the fiber incorporation zone; and a discharge head which is disposed downstream in the conveying direction. In this case, extremely gentle treatment of the plastic and the fibers takes place in the fiber incorporation zone so that the fibers are broken only to some minor extent, i.e. they are maintained in a randomly averaged range of length of 1.5 to 4 mm. It is of advantage if only a single mixing element is available on each shaft, i.e. a pair of mixing elements on both shafts, and subsequently a conveying screw element before the next pair of mixing elements.
The development according to which the discharge head comprises at least one die channel having a channel section which tapers continuously in the conveying direction and to the aperture angle a of which a xe2x89xa630xc2x0 applies, and according to which the tapering channel section is followed by a cylindrical channel section of a length l and a diameter d, the ratio l/d being great, and according to which l/dxe2x89xa75 applies to the ratio l/d, ensures that the glass fibers are enclosed in the plastic material and do not project there-from, which might give rise to considerable trouble during the subsequent treatment on injection molding machines. The preceding impregnation of the fiber rovings ensures a good and sufficient incorporation of the fiber sections into the thermoplastic, there being no need of a process of intimate kneading during which the fiber sections would be reduced to too small pieces.
Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing.